It is proposed that cytochromes and ferredoxins be isolated and characterized primarily from purple phototrophic bacterial species. Emphasis will be placed on marine and halophilic species. The occurrence, physical-chemical properties, kinetics, and amino acid sequences of cytochromes and ferredoxins will be used to further develop a detailed molecular basis for the comparative biochemistry of electron transport proteins. Greater effort will be devoted to the least understood structural and functional classes of protein. Towards this goal, the amino acid sequences of such proteins as Rhodospirillum tenue cytochrome c3, Rhodopseudomonas sulfidophila cytochrome c-556, Ectothiorhodospira halochloris cytochrome c-551, Ectothiorhodospira halophila and Rhodopseudomonas globiformis high redox potential 4-Fe-S ferredoxin (HiPIP), and Chromatium vinosum cytochrome c-533 (550) will be determined. The extent of interspecies transfer of bacterial electron transport protein genes is unknown, but will be evaluated through intensive study of species thought to be donors and recipients of such genes (a denitrifying strain of Rhodopseudomonas spheroides in particular). A molecular concept of the bacterial species based on cytochrome c2 amino acid sequences will be expanded to include cytochrome c'. The kinetics of oxidation-reduction of newly characterized HiPIP's should completely define the active site of electron transfer. The kinetics of ligand binding and sulfide oxidation in the flavocytochromes c combined with a study of acceptor specificity may clarify the functional role of this class of cytochromes. The possibility that photosynthetic bacterial cytochromes c3 might substitute for Desulfovibrio cytochromes c3 as electron acceptors and effectors in the hydrogenase-catalyzed oxidation of hydrogen gas will be tested. Hydrogenases from Desulfovibrio vulgaris and Chromatium vinosum will be compared with respect to their ability to donate electrons to cytochrome c3.